Alloy 625 is a solid-solution matrix-stiffened face-centered-cubic alloy at elevated as well as room temperatures. The strength of alloy 625 is derived from the stiffening effect of molybdenum and columbium on its nickel-chromium matrix. High tensile, creep and rupture strength; outstanding fatigue and thermal-fatigue strength; oxidation resistance; excellent brazeability and weldability are some of the properties of this alloy. However, alloy 625 has a very small hot working temperature range; and even though it is austenitic at room temperature, it is an inherently stiff material and difficult to move, especially in large section sizes. In general, alloy 625 has good mechanical and physical properties for use as a wear surface and is resistant to salt water corrosion making it excellent for use in sea water applications. The specific properties of the alloy 625 are reported in a brochure entitled "INCONEL alloy 625" by Huntington Alloys, Inc., Huntington, W.Va., a manufacturer of alloy 625. (INCONEL is a registered trademark of International Nickel Co.)
Initially, alloy 625 was developed as a sheet metal and had applications limited to sheet and tubing uses. Once it was demonstrated it could be successfully forged, it had other applications consistent with conventional forging techniques.
A proposed application of alloy 625 forgings is as a protective sleeve on marine shafting to keep sea water from corroding the steel surface of such shafting. The advent of increased and expanded undersea exploration has made it highly desirable to have available for use as a high strength, corrosion resistant material which can be fabricated into structures (large section size and large mass), and/or used in rotating equipment. The operating conditions under which these large undersea structures and/or rotating equipment are subjected demand high and exacting multidirectional properties.
Using conventional metal working methods, alloy 625 sleeves could be formed by rolling the material into a plate, forming the plate into a sleeve, and seam welding the plate. However, the seam weld would be a potential failure point for the sleeve. Prior to the present invention, all alloy 625 forgings were limited to small section size and small mass. Production of alloy 625 sleeves using forged parts employing conventional methods was directed to forging small or standard diameter ingots into rings. Large section, large mass sleeves formed from these rings required welding two or more forged rings together to form the final sleeve. These sleeves also suffered from the existance of welds in the finished product.
Huntington Alloy Inc. has produced large diameter remelted ingots of alloy 625 with a diameter of approximately 40 inches suitable for making large section, large mass forged seamless sleeves. However, there has been no process or method by which to work the material to form a large section, large mass forged sleeve from such ingots. The inherent stiffness of alloy 625, coupled with the very small hot working temperature range and the grain growth phenomenon of alloy 625 at elevated temperatures, made alloy 625 difficult to work to produce a large section, large mass forged products and still meet the high and directionally uniform mechanical properties desired in such a forged product.
Powder technology relating to hot isostatically pressing ("HIPping") alloy 625 has resulted in the production of small preforms not approaching the size of the workpieces from which the finished large section, large mass cylindrical sleeves of the present invention are produced. There has been great difficulty in producing HIPped alloy 625 powder preforms with sufficient properties that will result in the finished large section, large mass cylindrical sleeve having the desired properties after being worked. Even if the HIPped alloy 625 preforms were made of a sufficient size to be the starting workpieces for the large section, large mass cylindrical sleeves of the present invention, the workpieces would be difficult to work and still meet the high and directionally uniform mechanical properties desired on the finished forged product.
The technology developed by the present invention not only overcomes the problem of producing large section, large mass forged sleeves from large diameter ingots of alloy 625 and HIPped workpieces made from alloy 625 powder, but also provides the specific thermo-mechanical procedures developed to provide uniformly high mechanical properties, high ductility and a high fatigue limit in a product used in the corrosive sea water environment.